This chapter describes a general issue of selecting renewable energy sources (RES) and technical systems. To achieve the nearly zero-energy building (nZEB) standard, application of an RES (e.g., solar, wind, geothermal, hydropower, and biomass energy) is necessary. Each type of RES has specific characteristics and can be used to produce electricity and/or heat in certain systems. A short review of various systems using renewable energy sources is presented. To find the required and satisfactory solution that guaranties meeting the nZEB standard, an analysis must be carried out considering a number of aspects: local availability, structure and time-dependence of energy demand, building construction, economic conditions, legal regulations, and specific requirements. Finally, two examples of modernisation towards the nZEB standard are included.